Optical transmission systems employ Wavelength Division Multiplexing (WDM) to increase information handling of an optical fiber transmission line, typically a long haul transmission line. Early WDM systems operated with a relatively narrow wavelength bandwidth, centered around 1550 nanometers, e.g. 1530-1565 nanometers, referred to as the C-band. This is the wavelength region where standard silica based optical fibers have optimally low absorption.
In most WDM systems there is a trade-off between the number of channels the system accommodates and the channel separation. Higher bit rates generally call for an increase in channel spacing. Both goals favor a wide operating spectrum, i.e. a wide range of operating wavelengths.
Recently, systems have been designed that extend the effective operating wavelength range well above the C-band transmission band. In terms of wavelength, the new band, referred to as the L-band, is variously defined, but for the purpose of this description is 1570-1610 nanometers. Substantial work has also been done in the S-band, defined as 1460-1530 nm. Use of these added wavelengths substantially extends the capacity of WDM systems. There is an ongoing effort to further extend the effective operating wavelength window to above 1610 nm, for example to 1620 nm. Success of these efforts will depend on finding components that provide effective operation over this broad wavelength range.
An important aspect In the design of optical fibers for high bit rate, wide-band, systems, is management of chromatic dispersion. This problem grows significantly as the data bit rate is increased. Chromatic dispersion is the property of optic fiber that causes different colors of light to travel at different speeds and its effect in digital multiwavelength systems can be both positive and negative. When dispersion is severe, optical data pulses traveling over long transmission lengths may overlap. In early systems, efforts were made to reduce dispersion as much as possible. Initial attempts focused on making fibers with very low intrinsic dispersion. As optical transmission systems developed with higher bit rates, efforts shifted to correcting dispersion using dispersion compensating elements, typically lengths of optical fiber with dispersion values equal to and opposite those of the transmission line.
These elements, referred to as Dispersion Compensating Fiber (DCF) are very effective, but add cost to the system. Thus an optical fiber transmission line with high dispersion limits the distance that high bit rate signals can travel without expensive dispersion compensation.
On the other hand, too little dispersion allows adjacent signals to interfere and produce crosstalk.
The ideal long distance fiber has enough dispersion to suppress crosstalk, but small enough dispersion to allow high bit rate signals to travel long distances, and relatively the same amount of dispersion for each wavelength. These fibers, designed with deliberate finite dispersion values, are referred to as non-zero dispersion fibers (NZDF).
However, it is known that even in state of the art systems having dispersion compensating elements, some residual dispersion exists in the demultiplexed channels. This residual dispersion is easily compensated by a selected channel compensation component that, like the DCF described above, typically is a length of optical fiber having dispersion characteristics designed for the center wavelength of the channel. The dispersion of the channel dispersion compensating fiber (CDCF) is chosen to be equal to, but opposite in sign from, the residual channel dispersion.